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Basic MRI I. Chapter 2. The Magnet. The heart of the system Induces tissue magnetization Induces resonance with a specific frequency Frequency is proportional to field strength. Frequency is proportional to field strength. Freq = gyromagnetic ratio x field strength

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basic mri i

Basic MRI I

Chapter 2

the magnet
The Magnet
  • The heart of the system
  • Induces tissue magnetization
  • Induces resonance with a specific frequency
  • Frequency is proportional to field strength
frequency is proportional to field strength
Frequency is proportional to field strength
  • Freq = gyromagnetic ratio x field strength
  • Freq = “gamma” x B-naught
  • Freq = γHB0
  • Gyromagnetic ratio = γH = 43.58 MHz/T
  • Field strength = B0
magnetic field
Magnetic field
  • Magnetic fields have strength and direction – they are vectors
  • Strength measured in Gauss or in Tesla
  • 1 Tesla = 1 T = 10,000 Gauss = 10,000 G
  • 1000 G = 1 kG
  • Direction is defined as the “z-direction” in MR, in 3D Cartesian coordinates
gradients
Gradients
  • Intro to gradient concept
  • Linear gradients
  • Non-linear gradients
gradients1
Gradients
  • Orientation
    • Definition of orthogonal = “ at right angles”
  • Functions
    • To get images—all types
    • To get specialized images (gradient echo)
    • To reduce artifacts
  • Strength
    • Units of mT/m
    • High value = ~ 20 mT/m
gradients2
Gradients
  • Rise time = time for gradient to reach its maximum, like 0.20 msec
  • Slew rate = rate of gradient change, like 100 mT/m/msec
eddy currents
Eddy currents
  • Electrical currents induced or generated in conducting materials, in metal in the machine, minimized by gradients
shimming
Shimming
  • To achieve homogeneous field strength
shielding
Shielding
  • Passive—large pieces of ferromagnetic metal like iron
  • Active—electromagnets to decrease the extent of the field
rf system
RF system
  • Coils
    • Body coils
    • Surface coils
    • Special coils
      • TMJ
      • Head
      • Wrist
    • Linear, quadrature, CP, phased array
rf shielding
RF shielding
  • Farraday cage
    • To decrease stray RF energy from lights and motors outside the magnet from getting into the receive coil (antenna)
    • Copper screen
    • Sheet metal
computer
Computer
  • To store and acquire signals in k-space
  • To reconstruct images
  • To perform Fourier Transform
  • Storage, retrieval, transmission of images
  • Post Processing, zoom, magnification, contrast/windowing, 3D reconstructions